Cleanup crews blocked off parts of Buzzards Bay today with booms and worked to clear the remnants of an oil sheen left by a tugboat that ran aground, spilling at least 300 gallons of gear oil into the water, the Coast Guard said.

The 93-foot tugboat Justice was towed to the Massachusetts Maritime Academy from the spot near Stony Point Dike in Wareham where it ran aground at 12:01 a.m Thursday, a Coast Guard statement said. The spill shuttered the Cape Cod Canal from the early hours of the morning until 11:30 a.m.

The boat’s lower starboard drive, from which the fluid was leaking, was sheared off the bottom of the boat when it ran aground two miles south of the canal and still remains in the bay, said Adam Stanton, a Coast Guard spokesman.

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A Coast Guard statement said the drive was carrying 630 gallons of gear oil, a high-viscosity lubricant used in engines to protect gears, but most of the fluid remained in the boat and the submerged drive appeared to have stopped leaking.

“Gear oil is more persistent than fuel oil and response personnel are already looking to quickly clean up any pockets of oil found,” the Coast Guard said.

Christopher Reddy, a senior scientist at Woods Hole Oceanographic Institution who studies oil spills, said that unless the spill is much larger than reported, the environmental impact will likely be minimal.

“Is a bird going to get caught in the wrong place at the wrong time and get oiled? Possibly,” Reddy said. “Is the fishing industry going to collapse? No.”

Reinauer Transportation Companies, which owns the Justice, has located the propulsion unit on the bottom of the bay and is working in conjunction with the Coast Guard and US Army Corps of Engineers to recover it, a statement from the company said.

“Environmental safety is of the utmost importance to Reinauer — and care of the environment is paramount,” said Bert Reinauer, the company’s vice president. “As soon as we learned about the accident, our emergency team rushed to the scene with our environmental cleanup crew to contain and clean up the spill.”

The crew members on the Justice passed alcohol tests after the grounding, and are awaiting the results of a drug test, Reinauer said in the statement.

Booms have been deployed at the mouth of Buttermilk Bay and at Burgess Point to prevent the oil spreading to shellfish beds, and the Division of Marine Fisheries has closed some fisheries in the area, said Ed Coletta, a spokesman for the Department of Environmental Protection.

The exact size of the spill is not known, but the oil sheen spread throughout the southern end of the bay, the Coast Guard said. The good news: None of the vessel’s 38,000 gallons of fuel was spilled and the boat’s hull was undamaged, Stanton said.

In April 2003, a barge owned by Bouchard Transportation Co. ran aground and dumped 98,000 gallons of oil into Buzzard Bay. The company eventually paid $6 million to clean up the water and surrounding shore.